Sublimation transfer method has been widely used for preparing printed articles since it can provide high-quality images of excellent transparency, and high reproducibility and high gradation of neutral tints, which are equivalent to the conventional full-color photographic images. As the printed articles, digital photographs, and ID cards which are used in various fields and known as identification card, driver's license card, membership card, and the like, are known.
In the case of image forming in accordance with the sublimation transfer method, a thermal transfer sheet that has a dye layer provided on one surface of a substrate, and a transfer receiving article, for instance, thermal transfer receiving sheet that has a receiving layer provided on one surface of another substrate, is used. The dye layer of the thermal transfer sheet is superposed on the transfer receiving article, and then, they are heated by a thermal head from the back face of the thermal transfer sheet so as to transfer dyes of the dye layer onto the transfer receiving article, and thereby a printed article in which an image is formed on the transfer receiving article is obtained. According to the sublimation transfer method, since it is possible to control the transferring amount of the dye dot unit by dot unit with varying the amount of energy applied to the thermal transfer sheet, it is possible to perform a density gradation. Therefore, this method can provide a high quality image which is very vivid, and excels in the transparency, and the color reproducibility and the gradient of halftones, and which is comparable to full-color photograph image.
In recent years, on any other than the thermal transfer image-receiving sheet material to form a thermal transfer image needs to correspond to the printed product, it has been proposed that the intermediate transfer medium is provided for releasably receiving layer on a base material (for example, the Patent literature 1). According to the intermediate transfer medium, dyes of the dye layer of the thermal transfer sheet is transferred onto the receiving layer of the intermediate transfer medium to form a thermally transferred image, and then the intermediate transfer medium is heated from the back face thereof so as to transfer the receiving layer onto an arbitrary-selected transfer receiving article, it is possible to obtain a printed article where the thermally transferred image is formed an arbitrary-selected transfer receiving article.
In the printed article which is obtained by forming a thermal transfer image onto a receiving layer of the thermal transfer image-receiving sheet in accordance with the sublimation transfer system, or by forming a thermal transfer image onto a receiving layer of the intermediate transfer medium and retransferring the receiving layer onto an arbitrary-selected transfer receiving article, the receiving layer on which the thermally transferred image has been formed is located on the outermost surface of the printed matter. Although the thermal transfer image which is formed on the receiving layer in accordance with the sublimation transfer method is excellent in the formation of gradation image as mentioned above, the obtained printed article suffers with a problem of lack of durability including plasticizer resistance, abrasion resistance and solvent resistance, since the print is formed with dyes which each has a relatively low molecular weight, and they are different from the conventional printing ink which are made of pigments, and they does not have a vehicle.
Therefore, recently, a method of forming a protective layer, in which a protective layer transfer sheet having a protective layer is superposed on the receiving layer of the thermal transfer receiving sheet onto which a thermal transfer image has been formed, then the protective layer is transferred onto the receiving layer by using a thermal head or heating roll or the like, thus, the protective layer is formed on receiving layer of the thermal transfer receiving sheet onto which a thermal transfer image has been formed, has been known. It is possible to improve the durability of the image by forming such a protective layer on the image. With respect to the intermediate transfer medium, similar attempts have been made, for example, in the Patent literature 2, an intermediate transfer medium, where a peeling layer, a protective layer, and a layer functioned both as receiving layer and adhesive layer is provided on a substrate, has been proposed. According to this intermediate transfer medium, it is said that it is possible to impart durability to the thermal transfer image, since the protective layer is located on the surface of the receiving layer onto which the thermal transfer image has been formed, after the receiving layer onto which the thermal transfer image has been formed, and the protective layer are transferred on a certain substrate. Incidentally, in the protective layers which have been known to date, it has not attained to satisfy both of physical durability and chemical durability at the same time, therefore, there is a room for improving the protective layer so as to satisfy the both of durability at the same time.